The present application relates to an angular velocity sensor used for a camera shake detection in a video camera, a motion detection in a virtual-reality device, and a direction detection in a car navigation system, for example. More particularly, the present invention relates to a three-tuning-fork type angular velocity sensor provided with three vibrator arms.
As related art angular velocity sensors for general use, so-called vibration type gyrosensors have been widely used, in which a vibrator is allowed to oscillate at a predetermined resonant frequency and Coriolis force caused by the influence of an angular velocity is detected by a piezoelectric element or the like, thereby detecting the angular velocity. The vibration type gyrosensor has advantages of a simple mechanism, a short start-up time, and low manufacturing cost. For example, the gyrosensor is builted in electronic devices such as a video camera, a virtual-reality device, and a car navigation system, etc. In this case, the gyrosensor is utilized as a sensor for a camera shake detection, a motion detection, and a direction detection, etc., respectively.
With advancement of miniaturization and higher performance of electronic devices, the vibration type gyrosensors, which are built in such electronic devices, are also required for further miniaturization and higher performance. For example, to achieve multi-functionality of the electronic devices, there is such a demand that the gyrosensor is mounted on the same integrated substrate in combination with various types of sensors used for other purposes to achieve miniaturization. In such a miniaturization, it has become typical to use a fabrication technology called MEMS, in which a structure is formed by using a single-crystal substrate such as silicon (Si), and a thin-film formation process and a photolithography technology employed in a semiconductor manufacturing field (for example, see Japanese Patent Application Publication No. JP2005-241382, hereinafter, referred to as Patent Document 1).
Patent Document 1 discloses a cantilevered angular velocity sensor in which a drive electrode for excitation and a detection electrode for angular velocity detection are each formed on one surface of a single arm portion that forms a vibrator via a piezoelectric film. In the angular velocity sensor, the arm portion is excited in a direction vertical to a surface on which the piezoelectric film is formed, and a vibration component in a direction parallel to that surface is detected as the angular velocity.
In Japanese Patent Application Publication No. JP2006-17569 (hereinafter, referred to as Patent Document 2), there is disclosed a tuning-fork-type angular velocity sensor, in which a drive electrode for excitation and a detection electrode for angular velocity detection are each formed on one surface of each of two arm portions that form a vibrator via a piezoelectric film. In the angular velocity sensor, the arm portion is excited in a direction horizontal to a surface on which the piezoelectric film is formed, and a vibration in a direction vertical to that surface is defined as an angular-velocity detection direction. In Japanese Patent Application Publication No. JP2001-124561 (hereinafter, referred to as Patent Document 3), there is disclosed a three-tuning-fork type angular velocity sensor provided with three arm portions, each of which has a triangular cross-section and forms a vibrator. This angular velocity sensor is configured such that an arraying direction of the arm portions is an excitation direction and a vibration in a direction perpendicular thereto is defined as the angular-velocity detection direction.
However, in angular velocity sensors with the cantilever structure described in Patent Document 1, a rotational moment is generated in a root region of the arm portion when the arm is oscillated, and the resultant vibration is transmitted to a base portion that supports the arm portion, posing an issue that an angular-velocity detection characteristic is deteriorated.
Furthermore, in the tuning-fork-type angular velocity sensors described in Patent Documents 2 and 3, an excitation state of each arm portion, which is a basic mode, is configured such that each arm portion is vibrated and excited in a direction along the surface on which the piezoelectric film is formed. Thus, a center of rigidity of the vibrational excitation by the piezoelectric film is deviated from a center of gravity of the vibrator. Therefore, when a deviation occurs in the drive frequency due to superimposing of a disturbance signal, a vibration surface of the vibrator in a resonant state is easily deviated. As a result, even in a state that there is no angular velocity, detection output may changed. This may significantly increase noise